Getting C wire to Nest using a transformer

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Old 09-09-19, 09:59 AM
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Getting C wire to Nest using a transformer

I only have a two wire thermostat: Rc and Y.

Can someone please tell me how an external plug in transformer can power the Nest thermostat? I know one wire goes into the C terminal of the Nest. The other goes...where...doubled up with the existing Rc wire? This makes my brain call out for help. The thermostat has 24 v but the Nest steals power to charge its battery and this is resulting in the Nest not working. An external transformer seems to be one way of powering the Nest.

My brain tells me the red wire @ 24 v and the transformer wire @ 24 v=48 volts

How can this work then?
 
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Old 09-09-19, 10:04 AM
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Welcome to the forums.

Is this to run heat or A/c ?
Heat is normally two wires and A/C is three to four wires.
 
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Old 09-09-19, 10:22 AM
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It is air conditioning only. No heat.
 
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Old 09-09-19, 10:29 AM
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Most of the time it's the heat being used that requires the external 24v transformer. This may be a little experimental on your part. Connect the external 24vAC power to Rc and C and connect the A/C wiring to Rh and Y1.

If this doesn't work...... put the transformer on Rh and C and the A/C on Rc and Y1.

I hesitate to tell you to combine two wires, add a jumper and put it on Rc but if the above doesn't work.... this will be your next step.
 
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Old 09-10-19, 06:13 PM
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Thank you for your advice. I am going to try the transformer on Rh and C. I will let you know. I will put the red a/c wire on Rc, and he yellow a/c wire on Y1. I will check power at Rh and C, then Rc and Y. If I see there is power from the transformer, and the Nest is charging, and there is no power at Rc and Y but there is when I have the Rc and Y wires hooked up, I will feel better. I think it will work. All this for a C wire. It is not right. They should not sell something as compatible when the Nest and other smart thermostats need a C now, or later or sooner.

There is so much contradictory info on doubling up wires too as in adding a red wire from a transformer into a slot along with either an Rc or Rh wire and I won't do it because I think it is a double-feed situation. As the Nest website says in part:

"Instead of using jumper wires, Nest Learning Thermostats use an internal jumper between the Rh and Rc connectors. A Nest Learning Thermostat will automatically use this internal jumper if your system needs it.

Important: Never put more than one wire into a Nest thermostat connector. If your current thermostat has a jumper wire, do not enter it in the Nest app to get a wiring diagram or o​connect it to your Nest thermostat."
 
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Old 09-11-19, 12:33 AM
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Power stealing thermostats have always been a crap shoot. The problem is that the thermostat requires a certain amount of current flow in the circuit to charge the battery. If the stat uses too much current.... the circuit activates. Now with the use of electronic controls there is not always enough current flow. You've got a 50-50 chance it will work.

If you need to combine two wires...... you do that in a splice and use a single wire tail to connect to the terminal. Trying to get two wires in one terminal won't work.
 
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Old 09-12-19, 10:51 AM
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A Solution but not a pretty one
I got the Nest working for my two-wire central air system. It was not pretty and when people hear how it works, they will either buy me a beer or call me ignorant.

My Wiring That Did Not Work
To start, I added the transformer wires (I bought a transformer from Amazon) to Rh and C, while leaving Rc and Y connected on the air conditioning side of the thermostat mount. I got an E72 error meaning it "saw" all my physically connected wires but said I had no power at Rc. I removed the base and measured voltage. I had 24 v Rc to Y and 24 v from one transformer wire to the other.

My Wiring Change
I turned it all off, unplugged the transformer, unplugged the Nest and moved one transformer wire to Rc, left the other transformer wire in C, and moved the yellow wire over to the heat side W1, and the Rc wire over to the Rh terminal. When I snapped in the thermostat to its base, I plugged in the transformer, and I switched the breakers back on. The thermostat "liked" everything in that I had no error message and I could set it up. What the thermostat saw: RC powered up, a "heat" wire connected to W1, and a 24 v common. It did not sense a physical wire from the air conditioner, Rc, at all connected to Rh (it showed Rh as not in use, nothing there, no error about that).

The Final Step
I put the thermostat into heat mode, the thermostat say 70, I increased the heat setting to 74, and the air handler and air condenser turned on. It worked.

The Idiocy
At this time, my Nest works but switched to heat and of course this means "increasing" the temp to cool the house, not lower the temp. As I said, not pretty. I will continue experimenting

Summary
I am absolutely convinced no jumpers can be applied because with a transformer, it would be a double power feed along with an Rc or Rh wire but in my case anyway I only have a two-wire central air setup. Hook up a transformer to C and RH and your Red Rc and Y wires to the air conditioning side and no love. Hook up the transformer to C and RC and it senses itself as powered. It ignores the air conditioning system Rc wire in the Rh port as if it is not there physically even if it is there physically. Go figure. I may experiment a bit more but I have to say this product and setup are for the birds. If it were not for the rebates (I got two Nest thermostats for free, and two Google Assistants for free), I would have sent it all back. Too much time spent on task and an ignorant solution but who wants to buy me a beer?
 
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Old 09-16-19, 05:17 PM
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You can always break down and do what is not recommended but works. Pete mentioned above.

Leave the Y wire on Y1 as is.
Splice the other a/c wire (R) with one wire from the transformer.
Measure voltage between Y1 and other wire from transformer. If you measure 48vac then swap the two transformer wires.
Splice a short pigtail to the already spliced wires and connect to Rc.
Take the other transformer wire and put on C.

Some don't worry about checking for the 48vac as Nest is rated to tolerate the voltage on adjacent terminals.
 
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